1. Field of the Invention
The present invention relates to a physical quantity sensing device with a bridge circuit and more particularly relates to a sensing device that includes a pair of sensor elements, the impedance of which changes with a physical quantity to be measured. As used herein, the “physical quantity” is any quantity that can be measured with sensor elements, and refers to any of a broad variety of quantities such as forces (various forces including torque load), current, voltage, light quantity, and temperature.
2. Description of the Related Art
Magnetostrictive load sensing devices, including magnetostrictive sensor elements, have been developed for many years. A magnetostrictive sensor element is an element made of a magnetostrictive material, the initial permeability of which changes with the given load, and senses a variation in the initial permeability of the magnetostrictive material as a variation in the impedance (e.g., inductance and resistance) of a sensing coil, for example. Examples of preferred magnetostrictive materials include magnetic materials, soft magnetic materials and ultramagnetic materials such as ferrous alloys, iron-chromium based alloys, iron-nickel based alloys, iron-cobalt based alloys, pure iron, iron-silicon based alloys, iron-aluminum based alloys, and permalloys.
FIG. 1A is an equivalent circuit diagram showing a typical sensing circuit in a conventional magnetostrictive load sensing device. The bridge circuit shown in FIG. 1A has first and second input points N1 and N2, to which an AC voltage (or alternating current) is supplied, and first and second output points S1 and S2, which are connected to a differential amplifier (not shown). The AC voltage is supplied to the first and second input points N1 and N2 from an AC voltage generator 10.
In the bridge circuit shown in FIG. 1A, magnetostrictive sensor elements SE1 and SE2 are connected in parallel to each other. A bridge circuit of this type will be referred to herein as a “parallel bridge circuit”. Load sensing devices with such a parallel bridge circuit are described in Japanese Patent Application Laid-Open Publications Nos. 5-60627, 10-261128 and 2001-356059 and Japanese Utility Model Application Laid-Open Publication No. 5-45537, for example.
In a parallel bridge circuit, if the pair of magnetostrictive sensor elements thereof had significantly different initial permeabilities, then the equilibrium point under no load and the output sensitivity under load would be inconsistent so as to decrease the accuracy and reliability of the resultant sensor element value. That is why those unwanted effects, caused by such a significant difference in initial permeability between the magnetostrictive sensor elements, need to be reduced.
One of the most effective methods for reducing those effects caused by such a difference in initial permeability is to increase the amount of alternating current (i.e., exciting current) flowing through the bridge circuit.
However, when the conventional parallel bridge circuit is adopted, it is very difficult to increase the amount of the exciting current for the purpose of reducing the variation in the characteristic of the magnetostrictive sensor elements. The reasons are as follows.
In the parallel bridge circuit shown in FIG. 1A, the magnitude of the fixed resistance of the bridge circuit needs to be substantially equal to the resistance value (or impedance) of the magnetostrictive sensor elements SE1 and SE2 to expand the measurable load range (i.e., sensing range) thereof. A magnetostrictive sensor element normally has an impedance of about 100 Ω or less, and therefore, the magnitude of the bridge resistance is usually fixed at around 100 Ω. For that reason, it is difficult to further increase the impedance of the bridge circuit.
On the other hand, if the impedance variation that has been produced in the parallel bridge circuit affected the oscillator (not shown) of the AC voltage generator 10, then the output AC signal of the oscillator would have a varied oscillation waveform. To avoid this problem, an operational amplifier or any other suitable circuit component needs to be inserted between the oscillator and the parallel bridge circuit so as to function as a buffer amplifier. If such a circuit configuration is adopted, then the output AC voltage of the oscillator reaches the parallel bridge circuit by way of the operational amplifier. As a result, the impedance variation in the parallel bridge circuit does not affect the oscillator anymore. However, due to the performance limits of the operational amplifier, the amount of exciting current that can be supplied to the parallel bridge circuit is several tens of mA (milliamperes) at most. On top of that, current flows symmetrically in the parallel bridge circuit. That is why the amount of exciting current flowing through each of the two magnetostrictive sensor elements SE1 and SE2 decreases to half of the amount of exciting current that was supplied to the input points N1 and N2.
For these reasons, it is very difficult to significantly reduce the unwanted effects caused by the variations in sensor element characteristic by increasing the amount of exciting current flowing through each of the magnetostrictive sensor elements SE1 and SE2.
Also, if the magnetostrictive sensor elements have an extremely low impedance, then a resistor needs to be further inserted between the bridge circuit and the operational amplifier and the amount of current should be limited such that the operational amplifier would not cause output saturation. As a result, the voltage applied to the bridge circuit further decreases and the load sensing range becomes even narrower.
Meanwhile, when a bridge circuit is made up of magnetostrictive sensor elements, zero point adjustment needs to be carried out in order to compensate for the difference in initial permeability between the magnetostrictive sensor elements. In the parallel bridge circuit disclosed in Japanese Utility Model Application Laid-Open Publication No. 7-2943, the zero point adjustment is done by controlling the resistance values of variable resistors that are connected in series to the respective magnetostrictive sensor elements. However, it is not possible to strike a complete balance by such a zero point adjustment as will be described later.
Magnetostrictive sensor elements are excited with alternating current. Thus, to achieve complete balancing, the imbalance in impedance needs to be reduced to zero both in the real and imaginary parts alike. According to a conventional zero point adjustment method, however, the impedance imbalance can be eliminated from just one of the real and imaginary parts, not both. For that reason, even if the zero point adjustment is done so as to minimize the output voltage under no load, a residual voltage will always be generated, which then produces a difference in the output characteristic of the two magnetostrictive sensor elements. Besides, since the output voltage under no load is not zero, the output voltage will have a narrower dynamic range. As described above, the conventional bridge circuit cannot achieve the complete balancing and guarantees only insufficient measuring accuracy.